It is known that when a silver halide colour photographic material is colour-developed, a coupler reacts with an oxidized aromatic primary amine colour developing agent to provide an indophenol, indoaniline, indamine, azomethine, phenoxazine, phenazine or other similar dye, whereby a colour image is formed. In such a system, colour reproduction is conventionally conducted by the subtractive colour process and silver halide emulsions, which are selectively sensitive to blue, green and red light, respectively, and the corresponding yellow, magenta and cyan dye image forming couplers are used. To form a yellow dye image, an acylacetanilide or dibenzoylmethane coupler is usually used, to form a magenta dye image, a pyrazolone, a pyrazolobenzimidazol a cyano-acetophenone, or indazolone coupler is used, and to form a cyan dye image, a phenolic coupler such as a phenol or a naphthol is used.
Most conventional dye image forming couplers are four-equivalent couplers, i.e. they theoretically require development of four moles of exposed silver halide as an oxidizing agent to form one mole of dye by coupling. On the other hand, two-equivalent couplers, which have an active methylene group substituted with a group which is released when the coupler couples with an oxidized aromatic primary amine colour developing agent, require only development of two moles of exposed silver halide to form one mole of dye. Since two-equivalent couplers require only half the amount of silver halide to form a dye as compared with four-equivalent couplers, two-equivalent couplers have many advantages, for example, a decrease in the processing time of the light-sensitive materials and improvements in the photographic properties and reduction in costs due to a decrease in the thickness of the emulsion layers.
Several attempts have been made to provide two-equivalent couplers by modifying 5-pyrazolone couplers which have been conventionally employed as magenta dye-forming couplers. For instance, the 4-position of a 5-pyrazolone coupler can be substituted with a thiocyano group as described in U.S. Pat. Nos. 3,214,437 and 3,253,924, an acyloxy group as described in U.S. Pat. No. 3,311,476, an aryloxy group as described in U.S. Pat. No. 3,419,391, a 2-triazolyl group as described in U.S. Pat. No. 3,617,291 or a halogen atom as described in U.S. Pat. No. 3,522,052.
However, the use of these 4-substituted-5-pyrazolone couplers is accompanied by various disadvantages, namely that a marked colour fog is produced, the coupling reactivity is insufficient, the couplers are unstable and their coupling reactivities are lost in the light-sensitive materials during storage, or the preparation of the couplers is quite difficult.
The 5-pyrazolones in which the 4-position is substituted with an alkylthio group, an arylthio group or a heterocyclic thio group are also known, as described in U.S. Pat. No. 3,227,554. However, most of these known thio-substituted pyrazolone compounds have insufficient coupling reactivity with an oxidation product of a aromatic primary amine colour developing agent, and are difficult to use them in conventional colour photographic light-sensitive materials due to the adverse effects on photographic properties of the mercapto compounds which are formed as a result of the coupling reaction. Furthermore, the chemical stabilities of these couplers are insufficient.
The color forming sensitivity of the above described two-equivalent magenta couplers is not necessarily sufficient for increasing the sensitivity of color photographic light-sensitive material which is the latest technical subject. Therefore, it has been strongly desired to develop a technique for increasing the sensitivity of a magenta color image forming layer.
Recently, the knowledge that magenta couplers having an alkylthio coupling-off group have a particularly high activity and a high color forming sensitivity has been obtained as described in Japanese Patent Application (OPI) No. 62454/80 or U.S. Pat. No. 4,264,723. However, these couplers are still insufficient for obtaining highly sensitive color photographic light-sensitive materials. ("OPI" refers to "open to public inspection")
In order to further improve these properties, magenta couplers having an arylthio coupling-off group are developed as described in Japanese Patent Application (OPI) No. 35858/82 or U.S. patent application Ser. No. 291,886 and a technique of using a diffusion-resistant 1-phenyl-3-pyrazolidone derivative together with the above-described magenta coupler for the purpose of preventing the formation of magenta fog during color development is conceived. As a result, it becomes possible to obtain properties which are sufficient for practical use.